Radiofrequency (RF) ablation of cardiac and other tissue is a well-known method for creating thermal injury lesions at the tip of an electrode. Radiofrequency current is delivered between a skin (ground) patch and the electrode. Electrical resistance at the electrode-tissue interface results in direct resistive heating of a small area, the size of which depends upon the size of the electrode, electrode tissue contact, and current (density). Further tissue heating results from conduction of heat within the tissue to a larger zone. Tissue heated beyond a threshold of approximately 50-55 degrees C. is irreversibly injured (ablated).
Resistive heating is caused by energy absorption due to electrical resistance. Energy absorption is related to the square of current density and inversely with tissue conductivity. Current density varies with conductivity and voltage and inversely with the square of radius from the ablating electrode. Therefore, energy absorption varies with conductivity, the square of applied voltage, and inversely with the fourth power of radius from the electrode. Resistive heating, therefore, is most heavily influenced by radius, and penetrates a very small distance from the ablating electrode. The rest of the lesion is created by thermal conduction from the area of resistive heating. This imposes a limit on the size of ablation lesions that can be delivered from a surface electrode.
Methods to increase lesion size would include increasing electrode diameter, increasing the area of electrode contact with tissue, increasing tissue conductivity and direct mechanical penetration of the tissue by the ablating electrode/needle.
The electrode can be introduced to the tissue of interest directly (for superficial/skin structures), surgically, endoscopically, laparoscopically or using percutaneous transvascular (catheter-based) access. Catheter ablation is a well-described and commonly performed method by which many cardiac arrhythmias are treated. Needle electrodes have been described for percutaneous or endoscopic ablation of solid-organ tumors, lung tumors, and abnormal neurologic structures.
Catheter ablation is sometimes limited by insufficient lesion size. Ablation of tissue from an endovascular approach results not only in heating of tissue, but heating of the electrode. When the electrode reaches critical temperatures, denaturation of blood proteins causes coagulum formation. Impedance can then rise and limit current delivery. Within tissue, overheating can cause steam bubble formation (steam “pops”) with risk of uncontrolled tissue destruction or undesirable perforation of bodily structures. In cardiac ablation, clinical success is sometimes hampered by inadequate lesion depth and transverse diameter even when using catheters with active cooling of the tip. Theoretical solutions have included increasing the electrode size (increasing contact surface and increasing convective cooling by blood flow), improving electrode-tissue contact, actively cooling the electrode with fluid infusion, changing the material composition of the electrode to improve current delivery to tissue, and pulsing current delivery to allow intermittent cooling.
Needle electrodes improve contact with tissue and allow deep penetration of current delivery to areas of interest. Ablation may still be hampered by the small surface area of the needle electrode such that heating occurs at low power, and small lesions are created. An improved catheter with needle ablation is disclosed in U.S. Pat. No. 8,287,531, the entire disclosure of which is hereby incorporated by reference.
While needle electrodes improve tissue ablation, the structural integrity of a needle electrode may be compromised by steam pops arising from RF “arcing” between the needle electrode and adjacent conductive components of the catheter, including a tip electrode through which the needle electrode extends. When electrical conduction occurs between the needle electrode and the distal end of the tip electrode, the resulting cavitation or mini-shockwaves produced by the steam pops can cause premature wear and tear on the needle electrode that could lead to breakage and detachment from the catheter.
The “arcing” may be reduced by increasing the distance, especially the radial distance, between the needle electrode and the tip electrode. However, by increasing the distance, the formation of coagulum between the needle electrode and the tip electrode may increase despite surrounding blood flow that typically tends to minimize or prevent coagulum formation.
Accordingly, it is desirable for a catheter to have a distal tip configuration that increases distance, especially radial distance, between the needle electrode and the tip electrode, and provides irrigation between the needle electrode and the tip electrode, especially at the distal end of the tip electrode, to minimize the formation of coagulum. It is also desirable that the irrigation be supplied by a dedicated fluid path with sufficient pressure so as to avoid blood seepage into the catheter while minimizing the risk of trapped air bubbles. It is further desired that the irrigation be supplied circumferentially around the outer surface of the needle electrode for uniform cooling.